1. Field of the Invention
The present invention relates to a catalytic converter having two or more honeycomb bodies disposed in a casing tube, and to a method for its production.
Catalytic converters are used in particular to clean exhaust gases of internal combustion engines and are used primarily in motor vehicles. In order to achieve a large surface area, such converters generally include one or more honeycomb bodies that have many channels through which an exhaust gas can flow. Since such honeycomb bodies cannot be produced monolithically with an arbitrarily great volume, it has long been known to dispose a plurality of such honeycomb bodies one after the other in a casing tube. An interstice or intermediate space remaining between such honeycomb bodies can bring about an additional mixing under some circumstances and therefore can achieve additional advantages.
Although two or more honeycomb bodies in a single casing tube are shown in a great number of references, such as German Published, Non-Prosecuted Application DE 39 39 490 A1, which is the point of departure for the present invention, nevertheless until now it was hardly a trivial problem to economically manufacture such catalytic converters on a large scale. For various reasons to be discussed in detail below, catalytic converters with two or more disks were therefore first assembled (usually welded) at the very end of the manufacturing and coating process, so that the advantages of an integrated production and only a single part could not be fully exploited.
One reason is that producing a catalytic converter requires relatively many manipulation steps. It is not only necessary for the honeycomb bodies to be inserted into a casing tube, but various steps are also needed for applying brazing material, brazing, and later coating. Typical spirally wound honeycomb bodies, of the kind described in German Published, Non-Prosecuted Application DE 39 39 490 A1, have a tendency when axially strained toward mutual displacement of the winding layers, known as telescopes, before they are finally brazed together. Axial forces must therefore be avoided during production. Moreover, brazing in an upright position is not possible, because the axial forces of gravity that then arise necessarily cause a shift in position. Accurate positioning of the honeycomb bodies in the casing tube is not easily possible, either. Honeycomb bodies wound spirally from smooth and corrugated sheet-metal layers are not very elastic, because the smooth sheet-metal layers extend virtually in a circle, and therefore the elastic forces to be brought to bear by such a honeycomb body are inadequate to absorb the axial forces that occur in high-speed production processes.
However, from the prior art, namely Published European Applications No. 0 430 945 B1 and No. 0 279 159 B1, other forms of honeycomb bodies are also known, which have increased elasticity because of their special construction.
Published European Application No. 0 454 712 B1 also discloses honeycomb bodies with microstructures that extend transversely to the flow and that besides influencing the flow also bring about hooking together of the foils, so that a kind of form-locking connection between the layers is created that prevents mutual shifting. A form-locking connection is one which connects two elements together due to the shape of the elements themselves, as opposed to a force-locking connection, which locks the elements together by force external to the elements.